1. Field of the Invention
The invention relates to aqueous ink sets and their use as proof ink sets in the preliminary printing stage for offset or rotogravure printing for which purpose inkjet systems, in particular, also piezo drop-on-demand systems are used.
2. Description of the Related Art
The proof inks are used to produce so-called xe2x80x9cproofsxe2x80x9d, a preliminary printing stage which is binding with regard to color and is used prior to the offset or rotogravure printing process in order to represent preliminarily a future offset or rotogravure print as true to color as possible and corresponding to a color space which is defined by the employed colors. A possible basis for the offset color space is the so-called Japan Offset Standard or other comparable regional standards such as for example the Euroscale.
An important criterion for evaluating such xe2x80x9cproofsxe2x80x9d is thus the color space which is defined by the color locations of the primary and secondary colors of the inks of an ink set. For producing a valid preliminary printing stage or xe2x80x9cproofxe2x80x9d, certain conditions must be fulfilled, such as high resolution in the xe2x80x9cproofxe2x80x9d, i.e., at least 600 dpi for drop-on-demand methods, soft color courses, i.e., use of so-called light colors such as magenta and cyan, a color space as large as possible, wherein the color locations of the primary and secondary colors are as close as possible to the offset primary and secondary mixing space of the respective standard, as well as a high color stability of a print on special proof papers for universal use.
Aqueous ink sets or inks for drop-on-demand printing processes are known in principle. For example, U.S. Pat. No. 5,772,742 discloses an ink set for smooth and coated papers as well as transparent foils which has a high light fastness and has a high safety against bleeding of the colors at its print edges.
The German patent application 196 28 209 A1 describes aqueous ink sets which have a high light fastness and uniform or identical fading rates with regard to the individual as well as mixed colors.
Moreover, according to German document DE 692 05 760 T2 an ink set for ink jet printers is known which can be used for simple as well as coated papers and transparent foils. In addition to the minimal tendency of bleeding of the colors, such an ink set is said to have, inter alia, a good resistance against incrustations, a good stability, and a high color saturation.
The German patent application 197 34 241 A1 describes an ink set for use in ink jet printers where the colors are combined such that the bleeding of two inks into one another is reduced or prevented.
A disadvantage of these known inks or ink sets is that the aforementioned conditions for a preliminary printing stage as color-true as possible can be fulfilled only with great limitations or cannot be fulfilled at all.
Moreover, it was found to be disadvantageous that a print of a preliminary printing stage with inks according to the prior art often already feels dry to the touch but is, in fact, not yet dry.
During the drying process over the course of weeks, visible color shifts occur which on a short-term basis result in a wrong evaluation of the preliminary printing stage. A long drying process of the inks of the prior art is the result of a high contents of anti-drying agents which are added so that the jets of the printing system will not dry up and become clogged.
It is an object of the present invention to provide aqueous ink sets with soluble dyes whose use in inkjet systems, in particular, those using the drop-on-demand methods, will not exhibit the aforementioned disadvantages and which cover as much as possible at least one standard, for example, the Japan Offset Standard, and which quickly dry on printing media without special coatings, for example, media for use in piezo printers, provide stable color locations, and, moreover, remain light-fast over weeks in an interior space.
In accordance with the present invention, this is achieved in that the solvent is substantially water, that the inks contain at least one water-soluble dye as the essential colorant, wherein the first ink contains xe2x80x9cDirect Blue 199xe2x80x9d, the second ink contains xe2x80x9cReactive Red 180xe2x80x9d, and a third ink contains xe2x80x9cReactive Yellow 37xe2x80x9d, and an optionally present fourth ink contains xe2x80x9cAcid Black 194xe2x80x9d, either exclusively or as the primary dye, respectively.
The ink set is used primarily as a medium for short-term color-stable prints of all kinds. They are primarily used in piezo inkjet printers which operate according to the drop-on-demand method. Their primary use is as a proof ink set for producing proofs (test prints) as preliminary printing stages for offset or rotogravure printing.